1. Field of the Invention
The present invention relates to a gas expansion engine for use in a refrigerating cycle and, in particular, to a gas expansion engine having a cylinder and piston to define a variable-capacity expansion chamber therebetween.
2. Description of the Related Art
It is known in the prior art that, in a refrigerating cycle using helium as a refrigerant, a gas expansion engine is extensively used to obtain a low-temperature helium gas through the adiabatic expansion of high-pressure helium gas. In a reciprocating type gas expansion engine, a helium gas is adiabatically expanded within a variable-capacity expansion chamber defined between a cylinder and a piston and, at that time, released energy is recovered in the form of electric energy.
In this type of gas expansion engine, a motion conversion mechanism is connected to the piston to convert the reciprocatory motion to a rotation motion. This motion conversion mechanism ensures a stable reciprocatory motion of the piston and a generator for energy recovery is driven through the motion conversion mechanism.
When the piston is moved within a predetermined distance relative to a top dead point, a high-pressure helium gas is fed into the expansion chamber through the operation of a gas supply valve mechanism. The piston, after being moved past the top dead point, is driven, by a high-pressure helium gas, toward a bottom dead point. The capacity of the expansion chamber is increased with the movement of the piston toward the bottom dead point, causing a drop in temperature and pressure of the helium gas through an adiabatic expansion. With the movement of the piston past the bottom dead point and then the starting movement of it toward the top dead point, the low-temperature helium gas in the expansion chamber is discharged through the operation of the discharge valve mechanism. When the piston is moved within a predetermined distance relative to the top dead point, the discharge valve mechanism stops its operation and the supply valve again is moved. Through such a cycle, the low-temperature helium gas is intermittently sent out from the expansion chamber. The energy released from the high-pressure gas through the adiabatic expansion is recovered via the generator.
In a gas expansion engine using a refrigerating cycle, it is desirable to stably maintain a predetermined performance for an extended period of time. It is, therefore, necessary to achieve a structure where no undue force is exercised on those contact surfaces or slide portions of each part in the engine.
However, those severe conditions as will be set out below are inflicted on the gas expansion engine in terms of its specific characteristic. In the gas expansion engine, if a mechanically sliding part or parts are lubricated by an oil, grease, etc., a refrigerant gas is contaminated and the refrigeranting efficiency is lowered. For this reason, a sealing ring and guide ring as provided between the cylinder and the piston are employed without using any lubricating materials. Such sealing and guide rings without the use of any lubricating materials are worn in a short period of time in the case where any undue sliding movement, such as a one-sided impact, occurs at the associated parts. In order to suppress the wear of the sealing and guide rings, it is necessary that the piston be moved while these rings are smoothly slidably moved under a predetermined pressure below an allowable limit.
In the aforementioned reciprocating type gas expansion engine, however, the piston reciprocable in the cylinder has to be connected to the aforementioned motion conversion mechanism. In order to suppress the wear of the sealing and guide rings, it is required that the axis of the cylinder be aligned with the axis of the reciprocating part with an accuracy of the order of a few micron meters. In general, however, such an assembly is very difficult to achieve and, in the conventional gas expansion engine, the sealing and guide rings are liable to be worn out in a short period of time, bailing to maintain the engine performance over an extended period of time.
Further, the gas expansion engine parts need to be operated in the aforementioned lubrication-less way and hence the number of sliding parts need to be decreased to a minimum possible extent. In particular, the gas supply valve mechanism is required to be of an impact contact type without involving any slide motion. The use of the impact contact type mechanism undergoes a ready deformation of its constituent parts upon impact contact. If this deformation occurs, the valve operation becomes unstable for a short period of time. it is, therefore, not possible to stably maintain the engine performance for an extended period of time in view of the above.